HF links offer a beyond-the-horizon or BLOS (Beyond Line of Sight) capability which enables the implementation of long-distance or very long-distance communications without the need to use a satellite or relay stations. Frequency management enables the frequency or frequencies used for a given service to be chosen allowing the service concerned to be provided, while maintaining an availability of the other services and by adhering to the regulatory allocations, notably concerning maximum power and the radio pattern radiated outside the useful band. The prior art in the field of HF band frequency management corresponds to the mechanisms known as Automatic Link Establishment (ALE) and Automatic Link Maintenance (ALM). In the more general field of cognitive radio, there are finer and continuously applied frequency management mechanisms for selecting a range of frequencies or frequencies on the basis notably of the link budget and/or occupancy of the channels by other users.
According to a first approach, the prior art is essentially based on a static allocation of a frequency plan comprising all frequencies and drawn up during the task preparation, and second-generation or third-generation ALE protocols implemented during the link establishment. The conventional ALE protocol consists in searching for a passing frequency, by sequentially testing the different frequencies of the plan. According to the mode used, the selected frequency may be the first frequency meeting the link criteria, or the “best” frequency, i.e. the frequency which optimises the criterion in question for the frequency plan concerned. During the communication, link management protocol may need to modify the parameters of said link, including by again initiating a search for a suitable frequency, for example via an ALM mechanism. The conventional 2G or 3G allocation and corresponding link maintenance solution, “first come, first served”, does not allow any variable rights/priorities according to the service concerned to be taken into account.
According to a second approach, concerning the particular case of waveforms using a plurality of non-adjacent channels, it is possible to modify the choice of the frequencies during a communication, insofar as the same communications band is used, for example according to a principal described in the patent application of the applicant WO 2013/053703. These different solutions for creating and then managing the link are based on frequency allocations which are static, at least in the short term, and locally exclusive between the different networks. They enable the co-existence of the different transmission modes (single-channel, multi-channel) and also different services, such as telephony, telegraphy, e-mail, the Internet, file transfer, etc. Even if the possibility remains of making these frequency plans dependent on the time or season in order to adapt to the propagation conditions at a given moment, the allocation remains static and locally exclusive, which has the disadvantage of requiring a large number of frequencies overall. The broadband link management solution during a communication does not resolve the “first come, first served” problem. It does not allow different authorised maximum power levels to be managed according to the frequencies.
Moreover, according to a third approach, it is possible to compare towards the “cognitive radio” techniques which develop frequency-sharing principles in order in order to provide a solution to the problem of lack of frequencies. These principles are based on a frequency classification specific to the user and/or the service concerned. In the particular case of HF, three user cases can notably be distinguished: the exclusive primary user, the non-exclusive primary user and the secondary user. A secondary user is obliged not to use the frequency if it is being used by a primary user, and to release it if a primary user seeks to use it. However, the cognitive radio solution with frequency sharing between a plurality of users does not allow variable power levels to be specifically managed according to the frequencies. This solution assumes that it is possible to detect and identify other users and their type, and therefore to have access to their signalling, even though HF band communications are generally encrypted and more less protected.
The solutions offered by the prior art notably have severe limitations. They do not allow a set of allocations comprising a plurality of power levels, notably in the case of multi-channel transmissions, to be taken into account. A second difficulty appears when a frequency-sharing principle is to be combined with a joint management of single-channel and multi-channel transmissions. The basic principle of frequency sharing consists in allocating a set of services SLA (Service Level Agreement) to each channel with the allocation of certain characteristics for each SLA: power, primary or secondary usage type.
The choice of a set of channels for the implementation of a multi-channel transmission furthermore requires observance of certain constraints applying not only to the individual channels but also to the group of selected channels: maximum bandwidth, total transmit power, spectral transmission pattern. The prior art consists in combining a priori the channels meeting these constraints, which has the effect of dedicating them to the multi-channel waveform, and which goes against the frequency-sharing principle.
Existing solutions do not allow account to be taken easily of the fact of having to manage channels with different power levels while guaranteeing adherence to the associated patterns. Furthermore, existing solutions do not therefore allow the frequency allocator to supply sets of frequencies clearly larger than the sets of frequencies required for the current communication modes, where the allocations of the high-power frequencies are subject to international regulation and sharing at a worldwide level due to the very long-distance character of the HF propagation in ionospheric mode, since adherence to the limited powers cannot easily be guaranteed.
Each frequency has a certain number of authorised services or SLAs, a maximum authorised power level Pmax for each SLA service and associated usage type; exclusive primary, non-exclusive primary and secondary.